An inspection apparatus is known wherein a defect existing in a wafer is detected in such a manner that secondary electrons emitted when the surface of the substrate is irradiated and scanned with an electron beam are detected, wherein a wafer image data is produced on the basis of the detection result, and wherein the image data on each die on the wafer and the adjacent image data are compared with each other. A mapping-projection-type inspection apparatus is also well-known which obtains data on an image on a wafer in such a manner that the substrate is irradiated with a primary electron beam and secondary electrons emitted from the substrate are imaged by a secondary lens system.
The mapping-projection-type inspection apparatus is capable of irradiating a large area at a time. Therefore, such a system can have the number of scanning times markedly reduced and enables evaluation and observation of a sample at a higher throughput in comparison with the SEM system. An electron beam apparatus such as the mapping-projection-type inspection apparatus obtains an observed image by imaging, on a detector, through a mapping projection system, secondary electrons emitted from a sample. However, such a beam apparatus has a problem that the secondary electrons have a comparatively small energy of about several electron volts in the vicinity of the sample and therefore drift at the time of imaging under the influence of a surface potential difference due to charging, i.e., a potential difference caused by wiring conductors or the like and an insulating material existing between the wiring conductors, and a distortion in the observed image results.
On the other hand, reflected electrons, i.e., electrons reflected by the sample irradiated with the electron beam, have substantially the same energy as the incident energy, i.e., an energy of 2 to 3 keV, which is about 100 times higher than that of secondary electrons. For this reason, if reflected electrons are imaged by a mapping projection system, an observed image can be obtained which is not easily influenced by a surface potential difference and which has only a limited image distortion. However, the emission ratio of reflected electrons is much lower than that of secondary electrons. Therefore there is a problem that, in the case of detection of reflected electrons using a conventional detection system, e.g., one based on a combination of an MCP, a fluorescent screen and a CCD, the S/N ratio is not sufficiently high, it is necessary to increase an amount of electron beam current and a MCP gain, and, therefore, the electron source and the MCP are deteriorated in a short time period.
As regards secondary electrons, a primary electron beam irradiation energy and an efficiency σ of emission of secondary electrons, for example, in the case of irradiating an insulating material made of SiO2 with a primary electron beam are in a relationship shown in FIG. 1. As shown in the figure, when the irradiation electron energy is in the range from a lower limit of about 50 eV to an upper limit of 1500 to 2000 eV, the secondary electron emission efficiency σ is 1 or higher and more secondary electrons than the incident primary electrons are emitted. Therefore, the insulating material surface is positively charged up. When the irradiation electron energy is out of the above-described range, the secondary electron emission efficiency σ is lower than 1 and, therefore, the insulating material surface is negatively charged up.
If such charge-up is increased, a distortion is caused in an image formed by secondary electrons for observation and evaluation. In the case of defect inspection, for example, by comparison between images of adjacent dies formed on a device wafer, therefore, there is a defect misdetection problem, i.e., a problem that a false defect detection result is obtained.
As regards negative charge-up, a method has been proposed in which a capillary tube is used to locally supply a gas to an observation position on a sample whereby gas molecules collide with the sample surface and become ions by coupling with electrons to neutralize electric charge on the sample surface. However, such a method cannot supply a gas uniformly to the entire sample surface in the mapping-projection-type electron beam apparatus that radiates a beam through a wide area, and is not suitable for the mapping-projection-type electron beam apparatus.
As regards positive charge-up, a method is conceivable in which a sample is irradiated with electrons by a hot filament-type electron source such as tungsten to neutralize the charge-up. In such a case, however, the insulating material changes easily from a positively charged state to a charge-zero state and, further, to a negatively charged state. Such a transition is difficult to control.